This invention relates to electrochemical cells having a lithium anode and more particularly to a primary lithium electrochemical cell adapted for high reliability and high rates of current discharge.
Implantable cardiac defibrillators are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the defibrillator device continuously monitors the electrical activity of the heart of the patient, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore a normal heart rhythm. Shocks as large as 30-35 joules may be needed. Shocks are delivered from capacitors capable of providing that energy to the patient in a fraction of a second. In order to provide timely therapy to the patient after the detection of ventricular fibrillation, it is necessary to charge the capacitors with the required amount of energy in only a few seconds. Thus, the power source must have a high rate capability to provide the necessary charge to the capacitors, it must also possess low self-discharge in order to have a useful life of many months, and it must be highly reliable to provide an urgently needed therapy whenever necessary. In addition, since cardiac defibrillators are implanted, the battery must be able to supply energy from a minimum packaged volume.
One battery suitable for defibrillator use is disclosed in U.S. Pat. No. 4,830,940 to Keister et al, which patent is incorporated herein by reference. As disclosed therein, the anode material of the battery is lithium and the reactive cathode material is silver vanadium oxide. The anode is constructed in a serpentine-like fashion with cathode plates inserted between each of the convolutions thereof on both sides thereof. The electrolyte for a lithium battery or cell is a liquid organic type which comprises a lithium salt and an organic solvent. Both the anode and the cathode plates are encapsulated in an electrically insulative separator material. However, a disadvantage of this battery design is that the serpentine anode is not efficiently used since anode material at the bends is not faced by cathode material and is therefore not fully utilized. An improvement which addresses this problem is disclosed in U.S. Pat. No. 5,147,737 to Post et al, in which the active material on the serpentine-type electrode is positioned so that the sections of the serpentine-like structure which do not face cathode plates do not contain anode active material. However, the serpentine bends of the anode are still present to the detriment of volumetric efficiency. Additional problems with these battery designs include the number of piece parts and connections required to make the battery which can affect both the manufacturability and the reliability of the battery; and the difficulty of achieving good current distribution and utilization of reactive material due to the unmatched configurations of the anode and cathode.
Conventional lithium batteries can also employ an electrode body in which anode and cathode elements are combined in spiral wound form. A strip sheet of lithium or lithium alloy comprises the anode, a cathode material supported on a charge collecting metal screen comprises the cathode, and a sheet of non-woven material separates the anode and cathode elements. These elements are combined and wound to form a spiral. Typically, the battery configuration for such a wound electrode would be cylindrical. For example, such configurations can be found in U.S. Pat. Nos. 3,373,060; 3,395,043; 3,734,778; 4,000,351; 4,184,012; 4,332,867; 4,333,994; 4,539,271; 4,550,064; 4,663,247; 4,668,320; 4,709,472; 4,863,815; 5,008,165; 5,017,442; and 5,053,297. Unlike the battery of the '940 patent, there need not be anode material which is not mated to cathode material. Such designs therefore have the potential for an improved match between the cathode and anode components and improved uniformity of anode and cathode utilization during discharge. However, cylindrical cells would not achieve the same space utilization inside the case of an implantable defibrillator as a prismatic cell shape.
It has also been known to adapt wound electrodes to a prismatic case configuration by departing from a true spiral winding. For example, U.S. Pat. No. 2,928,888 discloses in FIGS. 5a and 5b therein an oblong electrode assembly wound on an elongated mandrel for use in a rectangular case. Also, for example, U.S. Pat. No. 4,051,304 discloses in FIG. 2 therein another oblong wound electrode assembly for use in a rectangular case. However, these patents do not indicate that such structures could be advantageously used for a high current rate capability lithium battery or that they provide a uniform utilization of reactive anode and cathode material during discharge.
Since the battery separator also occupies a significant volume in a wound cell, it would increase the space available for active material (and therefore increase the energy density for the cell) if the separator volume could be minimized while retaining its functionality and an adequate margin of safety. In U.S. Pat. No. 4,610,081 issued to Triner, a metallic bipolar battery plate is disclosed in which the anode material contacts one surface of the plate and the cathode contacts the other. On the anode side of the plate, the anode is pressed onto the plate and, in a preferred embodiment, a separator material is applied to the anode material prior to the pressing operation so that the separator is pressed into the anode plate in a single operation. However, it is not disclosed whether a pressed anode/separator could be used in a wound electrode.
It is therefore an object of the present invention to provide a high current rate capability lithium battery having a coiled electrode suitable for use in a prismatic case.
It is also an object of the present invention to provide a high current rate capability lithium battery which provides improved energy density.